An automotive vehicle typically includes an internal combustion engine containing a rotary crankshaft configured to transfer motive power from the engine through a driveshaft to turn the wheels. A transmission is interposed between engine and driveshaft components to selectively control torque and speed ratios between the crankshaft and driveshaft. In a manually operated transmission, a corresponding manually operated clutch may be interposed between the engine and transmission to selectively engage and disengage the crankshaft from the driveshaft to facilitate manual shifting among available transmission gear ratios.
On the other hand, if the transmission is automatic, the transmission will normally include an internal plurality of automatically actuated clutches adapted to dynamically shift among variously available gear ratios without driver intervention. Pluralities of clutches, also called clutch modules, are incorporated within such transmissions to facilitate automatic gear ratio changes.
In an automatic transmission for an automobile, anywhere from three to ten forward gear ratios may he available, not including a reverse gear. The various gears may be structurally comprised of inner gears, intermediate gears such as planet or pinion gears supported by carriers, and outer ring gears. Specific transmission clutches may be associated with specific sets of the selectable gears within the transmission to facilitate the desired ratio changes.
Because automatic transmissions include pluralities of gear sets to accommodate multiple gear ratios, unnecessary friction or parasitic drag is a constant issue; the drag arises from mechanical interactions of the various parts employed. Much effort has been directed to finding ways to reduce friction drag within automatic transmission components and systems.
By way of an example, one of the clutch modules of an automatic transmission associated with first (low) and reverse gear ratios may be normally situated at the front of the transmission and closely adjacent the engine crankshaft. The clutch may have an inner race and an outer race disposed circumferentially about the inner race. One of the races, for example the inner race, may be drivingly rotatable in only one direction. The inner race may be selectively locked to the outer race via an engagement mechanism such as, but not limited to, a roller, a sprag, or a pawl, as examples. In the one direction, the inner race may be effective to directly transfer rotational motion from the engine to the driveline.
Within the latter system, the outer race may be secured to an internal case or housing of an associated planetary member of the automatic transmission. Under such circumstances, in a first configuration the inner race may need to be adapted to drive in one rotational direction, but to freewheel in the opposite direction, in a condition referred to as overrunning. Those skilled in the art will appreciate that overrunning may be particularly desirable under certain operating states, as for example when a vehicle is traveling downhill. Under such circumstance, a driveline may occasionally have a tendency to rotate faster than an associated engine crankshaft. Providing for the inner race to overrun the outer race may avoid damage to the engine and/or transmission components.
In a second configuration, such as when a vehicle may be in reverse gear, the engagement mechanisms may be adapted for actively engaging in both rotational directions of the inner race, thus not allowing for an overrunning condition in a non-forward driving direction.
Above certain thresholds of rotational speed, need for interaction of the engagement mechanisms, particularly those associated with the first (low) and/or reverse gear ratios, may become unnecessary. Thus, rather than contributing to drag, for example at highway speeds, there may be substantial motivation to reduce and/or avoid interaction of the engagement mechanisms with any of the clutch parts, particularly those associated with the low/reverse clutch module.